added custom theme for sandbox so site theme does not interfere
using sandbox theme for consistency when the site theme is changed
a few improvements to theme sandbox - mostly style
using sandbox consistently rather than preview
improved navigation sandbox - not floating the menus based on whatever the current theme is
removed site menu toggle
cleaned up javascript component sandbox
fixed display of site menu
adds structural layouts
Merge pull request #5827 from cash/sandbox_layouts

Improves the Theme Sandbox in developers plugin